Crude oil distillation unit is the first in which petroleum crude is processed. The brief description of the methods used for crude distillation based on the prior art Miguel Bagajewicz and Shuncheng Ji, Rigorous Procedure for the Design of Conventional Atmospheric Crude Fractionation Units. Part I: Targeting, Ind. Eng. Chem. Res. 2001, 40, 617-626; Shuncheng Ji and Miguel Bagajewicz, Design of Crude Fractionation Units with Preflashing or Prefractionation: Energy Targeting, Ind. Eng. Chem. Res. 2002, 41, 3003-3011; Shuncheng Ji, Katy Tex. (US), Miguel J. Bagajewicz, Norma, Ok (US), Methods for increasing distillates yield in crude oil distillation, 2007, U.S. Pat. No. 7,172,686B1; Massimiliano Errico, Giuseppe Tola, Michele Mascia, Energy saving in a crude distillation unit by a preflashimplementation, Applied Thermal Engineering 29 (2009) 1642-1647; S. W. Golden, Prevent preflash drum foaming, Hydrocarbon Process. 76 (1997), 141-153. [U.S. Pat. No. 7,172,686B1, Ji and Bagajewicz, 2002; Errico, M., et. al. 2009, Bagajewicz and Shuncheng, 2001] are described below. Petroleum crude is first heated in a heat exchanger network using the hot products and pump around streams before entering a desalter. Water is either mixed with crude or fed to the desalter where most of the water soluble salt is removed. The desalted crude enters another heat exchanger network and receives heat from hot streams. The preheated crude then enters either to prefractionation column or preflash drum or furnace or combination of their off.
In case of prefractionation column, the top product of controlled distillation temperature range is routed to naphtha stabilizer directly. The vapor from the flash drum which requires further processing in main crude distillation column is either routed to column at any location above the flash zone or mixed to the liquid portion drawn from the flash drum before/after the furnace. The combinations of more than one preflash drum in series are also available in the prior art. In one of the combination, the heated crude is fed to flash drum where vapor and liquid are separated. The liquid from the flash drum is routed to furnace. The vapor from this flash drum is cooled in cooler and fed to the second flash drum. The vapor from second flash drum is then mixed with the liquid coming from the first flash drum before furnace. The liquid from the second flash drum is routed to the column at desirable location or to the side striper. In yet another combination reported in prior art, the vapor from the second flash drum, described above, is further cooled in the cooler and fed to yet another third flash drum. Vapor from third flash drum is mixed with crude coming from the first flash drum before furnace. The liquid from second and third are routed to the column at different location above the flash zone.
The crude coming from desalter/bottom of the flash drum/bottom of prefraction column is partially vaporized in the furnace and fed to the flash zone of the atmospheric column. The vapor from the flash zone moves upward in the column whereas liquid falls downwards to the bottom of column. The vapor is then fractionated into distillate products such as, not limited to, naphtha, kerosene, gas oil in upper section of the column. To recover the heat at different temperature level, several pump-around circuits along the column, where liquid streams are withdrawn, cooled, and sent back to upper trays are used along with the overhead condenser. The distillate products withdrawn from different trays of the column are then stripped by steam in their corresponding side strippers for removing lighter components to meet the products ASTM distillation specifications. The liquid falling downwards to the bottom from the flash zone is stripped using the steam.
Crude distillation unit is the largest and huge energy intensive among all petroleum processing units. The distillate products obtained from atmospheric distillation are more valuable than residue. Moreover, the demand of gas oil fraction is also increasing continuously worldwide. In view of this, designing a crude distillation unit for maximum distillate yield, particularly of gas oil, with minimum energy consumption could be main objectives of any designer. There are traditional techniques for increasing the distillate yields, which has its own limitations, such as increasing bottom stripping steam which is limited by increase in energy consumption, two phase formation (water saturation limit) at the plate of distillation and size (diameter) of the column, lowering the pressure of flash zone which is also limited by acid gas dew point temperature at top of the column, increasing furnace coil outlet temperature which is also limited by cracking characteristics of crude and increased energy consumption. The application of the flash drum and prefractionation column with main distillation column is reported in prior art to provide some energy savings. However, the major drawbacks of preflash and main distillation column combination is lower distillate yields generation in comparison to using the single main column due to loss of carrier effect of lighter fraction removed from the crude during the preflashing and requirement of significantly more space (diameter) and energy to process this additional distillate in vacuum column. However, in the preflash and distillation column integrated system, the distillate products in which there will be change in the yields depends on number of variables such as type of preflash i.e preflash drum or prefractionation column, feed temperature to preflash device, type of crude and vapor entry location in column [Ji and Bagajewicz, 2002; Errico, M., et. al. 2009, S. W. Golden, 1997]
Accordingly, still there is need for new method for improvement in the prior art of the crude distillation for increasing the distillate yield particularly, of gas oil to meets its increasing demand in gas oil driven society, energy savings for reducing green house gas emission and operating cost, and decreasing the yields of lower value distillates and residue by better fractionation in distillation column to increase the unit profit margin. Therefore, the innovation that can overcome these challenges described above will be of great importance. The importance of the innovation will be further augmented if; it can be implemented in a number of existing refineries economically and easily.